Generally the existing process for manufacturing products of flanged hollow frames is as follows.
First, a circle is punched out from a sheet of plate. Second, by reducing this circular plate placing it horizontally on the press, a flanged cylinder is formed. Third, the bottom of this cylinder is punched out and the edge remained at the circumferential edge of the punched bottom hole is straightly expanded or removed. In addition, the flange section formed with the preceding process is cut to the prescribed flange size and is further finished to the prescribed flange shape via bending work.
Therefore, with the existing process, material loss due to the trimming of flange and bottom sections as mentioned above and due to making allowance for reducing work, particularly when reducing eccentric hollow frame has made the process noticeably disadvantageous, and the requiring of much time and effort for working has been uneconomical. Moreover, with the preceding process, not to mention of eccentric hollow frames, manufacturing of those tapering in cylindrical section is further beset with difficulties.